In a soft X-ray region, light is significantly absorbed in all substances and in addition, a refractive index approximates 1. Hence, it is impossible in principle to make use of a lens operation due to its refraction. To that end, an optical system is made up of a mirror. In such a case, any general reflector made of a single-layer film has a direct incidence reflectivity of almost 0 and thus is far from functioning in the optical system. In contrast, a multilayer film made of a material relatively low in light absorption can function as a direct incidence mirror optical element, making it possible to use a reflective optical system realized by the utilization of effects thereof.
Heretofore, a reflector has been developed in order to obtain the aforementioned optical element such as a mirror, the reflector being made of a multilayer film including ultra-thin films formed by alternately stacking two kinds of substances, that is, A and B, up to at least several ten layers and defining a number of reflective surfaces as interfaces therebetween, with a thickness of the wavelength order that is determined based on an optical interference theory that phases of reflection waves from the respective interfaces match with each other. In order to obtain a high reflectivity, it is necessary to choose a proper combination of two substances, that is, the two substances A and B each having as small absorption coefficient as possible and having reflective indexes nA and nB, respectively with a large difference therebetween. As a substance pair that enables the highest reflectivity within an incident wavelength range of 11 nm to 14 nm in a soft X-ray region, an alternate multilayer film made of Mo and Si is exemplified (see Japanese Patent No. 3101695, for example). The multilayer film is formed by a thin film formation technique such as magnetron sputtering, EB evaporation, or ion beam sputtering.
When the multilayer film made of Mo and Si is formed with the above technique, a central portion and peripheral portion of a film that is being formed are different in temperature, leading to a temperature variation. This causes a difference in how diffusion proceeds in the multilayer film, leading to an uneven film.
Further, a soft X-ray mirror made up of the multilayer film only reflects an X-ray having such a wavelength as to meet a reflection condition from among incident soft-X rays and absorbs almost all the X-rays having other wavelengths than the above wavelength. In short, if an intense soft X-ray enters the multilayer film mirror, an energy of the X-ray absorbed in the multilayer film heats the multilayer film. The temperature rise of the multilayer film is supposed to reach about several hundreds of 0° C. in the case of synchrotron radiation, although depending on an intensity of a soft X-ray incident on a reflector or an absorptivity for the X-ray of the multilayer film. Therefore, when the multilayer film is heated, its fine structure is broken and changed.
In general, when a considerably uneven multilayer film is used or the fine structure of the multilayer film is broken, the reflectivity thereof drops. To prevent such a situation that heat is locally accumulated in the multilayer film or its structure is broken, the heat should be dissipated from the inside of the multilayer film through heat transfer. A soft X-ray mirror made up of such a multilayer film is set in a vacuum when used as a component of an exposure device. In a vacuum, the heat transfer due to gases does not occur unlike the use in the air. Hence, the heat of the heated multilayer film needs to be transferred to a substrate through the multilayer film.
To that end, there has been proposed a method of forming a multilayer film mirror that easily induces heat transfer. With conventional techniques, heat is dissipated from a multilayer film by providing a cooling mechanism for cooling the multilayer film (see Japanese Patent Application Laid-Open No. 05-119208, for example). Also, the following method has been developed. That is, a substance of a high heat conductivity (heat transfer layer) is provided in contact with a multilayer film and heat is dissipated from the multilayer film (see Japanese Patent Application Laid-Open No. 05-333199, for example).
However, in the aforementioned conventional methods of dissipating heat from the multilayer film, heat radiation property of the multilayer film itself is not taken into account. Unless the heat conductivity of the multilayer film itself increases, the heat is not released from the multilayer film.